Sodium hypochlorite is a commodity chemical product that is widely used for numerous industrial and individual purposes. It is generally prepared by reacting sodium hydroxide and chlorine and the product hypochlorite is provided in aqueous solutions of differing concentration. In many conventional hypochlorite production systems, a chlor-alkali plant is first used to produce the NaOH and chlorine reactants for subsequent production of hypochlorite. Sodium chloride is a co-product in the preparation and is thus present to some extent in the hypochlorite products. Lower concentration hypochlorite solutions are more easily prepared, but are less efficient to store and transport. Higher concentration hypochlorite solutions are preferred for storage and transport purposes, but are more difficult to prepare. Consequently, an effort continues to develop improved processes and systems to more easily and more efficiently produce high concentration or high strength hypochlorite solutions. Further, the hypochlorite decomposes slowly in the presence of sodium chloride and thus it is generally desirable to reduce the amount of sodium chloride co-product present in order to increase the shelf life of these solutions.
U.S. Pat. No. 4,780,303 is exemplary of developments in the field and discloses two stage processes for the continuous preparation of high concentration sodium hypochlorite. The processes comprise reacting in two stages pure chlorine or chlorine diluted with inert gases with a sodium hydroxide having a concentration of from about 20 to about 50 weight percent. The first stage of chloration of the sodium hydroxide is carried out in an absorption column in which the concentration of sodium hypochlorite and sodium hydroxide are respectively limited to 15% and 4.5% by weight. In the second stage, a crystallizer is continuously fed with the sodium hypochlorite solution from the absorption column, 48-50 weight percent sodium hydroxide, and the pure or diluted chlorine. At the outlet of this crystallizer, there is continually removed, generally by overflow, about a 25 weight percent sodium hypochlorite solution saturated with about 9.5 weight percent sodium chloride and containing from 0.3 to 0.8 weight percent sodium hydroxide in excess to avoid the formation of the chlorate, NaClO3. The sodium chloride formed in this second step is removed either discontinuously or continuously at the bottom of the crystallizer. The mean diameter of the sodium chloride particles is about 400 to 500 microns, thus permitting a very good solid-liquid separation and a very small mother liquor retention by the solid.
Further, WO2006/017234 discloses a two stage process and apparatus for producing hypochlorite (bleach) products of high strength and low salt. The high-strength, low salt bleach produced by the disclosed process and apparatus, when diluted to a lower strength comparable with typical domestic commercial bleaches has improved stability, and hence extended half-life when compared to such bleaches. The disclosed method is for the continuous manufacture of a high-strength, low-salt, aqueous sodium hypochlorite bleach from a lower strength aqueous sodium hypochlorite bleach from a first stage that has some sodium hydroxide and is essentially free of sodium chloride crystals. In the second stage, the method comprises continuously reacting in a tank 1) the lower strength aqueous sodium hypochlorite bleach that has some sodium hydroxide and is essentially free of salt crystals, 2) aqueous sodium hydroxide solution having a concentration by weight within a range from about 45% to about 51%, and 3) chlorine in gas and/or liquid phase that may or may not include inerts to create a solution having a) a precipitation zone where salt crystals are precipitating out of solution and falling downward to form a slurry, some of which is withdrawn, then cooled, and then re-introduced into the precipitation zone, and b) above the precipitation zone, a crystal-free mother liquor zone consisting essentially of crystal-free mother liquor containing a weight percentage of sodium hypochlorite greater than that of the lower strength aqueous sodium hypochlorite bleach being reacted.
Further still, WO2008/082626 discloses an improvement for simplifying the equipment and process that are the subject of the aforementioned WO2006/017234. A similar higher-strength, lower-salt bleach is produced (e.g. after removal of solids, a bleach having approximately 30% to approximately 35% by weight sodium hypochlorite and a NaCl/NaOCl ratio of about 0.21 to about 0.25 at 30% strength and about 0.10 to about 0.15 at 35% strength, with slight excess caustic). However here, the continuous process is conducted in a crystallizer tank without continuously drawing off mother liquor at the top of a calming zone. That allows the tank to not have a skirt baffle that otherwise would divide the portion of the tank above the bottom zone into a central inner zone surrounded by an outer calming zone.
Despite the numerous efforts in the field to date, there still remains a need however for ever greater efficiency with regards to the production of high strength, low salt alkali hypochlorites, and especially sodium hypochlorite.